A solid oxide fuel cell is a fuel cell employing as an electrolyte a solid electrolyte having oxygen ion conductivity, and attracts attention as a clean energy since the electrochemical reaction which causes electromotive force is a hydrogen oxidation reaction, and no carbon dioxide gas is formed. A solid oxide fuel cell usually has a stack structure comprising single cells each comprising an air electrode as an oxide, a solid electrolyte and a fuel electrode connected by an interconnector. Its operating temperature is usually about 1,000° C., and decrease in the temperature is attempted and practically employed by various studies, however, it is at least about 600° C. and is still high temperature.
Due to the cell structure and the high operating temperature of a solid oxide fuel cell, an air electrode material constituting the air electrode is basically required to have such properties that (1) it has a high oxygen ion conductivity, (2) it has a high electron conductivity, (3) its thermal expansion is similar to or about the same as that of an electrolyte, (4) it has high chemical stability and has high compatibility with other constituting materials, and (5) the sintered product is required to be a porous product and it has a certain strength, etc.
As a material of an air electrode which satisfies such properties, a composite oxide represented by (La1-xSrx)aCoyFe1-yO3 (hereinafter sometimes referred to as LSCF) having a perovskite structure is energetically studied and developed as an air electrode material excellent in the electrode activity.
For example, Patent Document 1 discloses a ceramic powder containing as the main component a lanthanum ferrite perovskite oxide. Specifically, it discloses a ceramic powder represented by a compositional formula (L1-xAEx)1-y(FezM1-z)O3+δ, wherein L is one or more of elements selected from the group consisting of rare earth elements such as La, Sc and Y, AE is one or two of elements selected from the group consisting of Sr and Ca, M is one or more of elements selected from the group consisting of Co, Mg, Sc, Ti, V, Cr and Ni, 0<x<0.5, 0<y≦0.04 and 0≦z<1 (claims).
And, specifically disclosed as a method for preparing the ceramic powder is to mix and pulverize lanthanum oxide, strontium carbonate, cobalt oxide and iron oxide in a solid phase using e.g. a mortar (hereinafter sometimes referred to as a solid phase method) and calcine the mixture (paragraphs [0032] and [0092] to [0094] (Example 1)).
However, by such a solid phase method, so long as four types of raw material element-containing particles are pulverized and mixed in a solid phase, it is difficult in principle to obtain one having a completely uniform composition at the micro level.
Further, Patent Document 1 discloses in Examples 2 to 3 and 6 to 11 an example in which (La0.6Sr0.4)1-z(Co0.2Fe0.8)O3+δ (y=0, 0.02, 0.04) having a specific surface area of 4 m2/g prepared by a citrate method, in addition to a solid phase method, is wet mixed with ethanol and then pressure-molded. This method is the method disclosed in the after-mentioned Comparative Example 1, and by this method, raw material powders are mixed in a solution of citric acid alone, La2O3 as one of the raw material powders and a citrate after the reaction are not sufficiently dissolved, and the system is in a slurry state (hereinafter this method will sometimes be referred to as a slurry method).
Further, Patent Document 2 discloses an air electrode material powder for a solid electrolyte fuel cell, which is a perovskite composite oxide powder represented by the formula ABO3, wherein the A site comprises at least one element selected from the group consisting of La and rare earth elements, and at least one element selected from the group consisting of Sr, Ca and Ba, and the B site comprises at least one element selected from the group consisting of Mn, Co, Fe, Ni and Cu, which is fine with an average particles size of at most 1 μm, and which has a narrow particle size distribution (claims).
Patent Document 2 relates to a LSCF powder having a small particle size and having a small dispersion of the particle size distribution, and to prepare such a powder, water-soluble nitrates of La, Sr, Co and Fe as raw material elements are dissolved in a predetermined proportion in water, NH4OH is added thereto to coprecipitate the respective insoluble salts, and the precipitates are dried and fired (hereinafter sometimes referred to as a coprecipitation method) (paragraph [0032]).
Since in this coprecipitation method precipitates are formed from a uniform solution, it is apparently considered that one having a uniform composition is easily formed, however, according to studies by the present inventors, in practice, the precipitates do not have a uniform composition since the pH at which insoluble salts of the respective elements precipitate and their crystal growth rates are different among the nitrates of the four types of elements. For example, a salt of one element is precipitated first and grows into large particles, and then micro crystals of the next element are precipitated on the large particles, and accordingly it is considered difficult in principle to obtain precipitates having a sufficiently uniform composition.